Review the regulation and actions of aldosterone.
n adults the adrenal glands emerge as fairly complex endocrine structures that produce two structurally distinct classes of hormones: steroids and catecholamines. The catecholamine hormone epinephrine acts as a rapid responder to stresses such as hypoglycemia and exercise to regulate multiple parameters of physiology, including energy metabolism and cardiac output. Stress is also a major secretagogue of the longer-acting steroid hormone cortisol, which regulates glucose utilization, immune and inflammatory homeostasis, and numerous other processes. In addition the adrenal glands regulate salt and volume homeostasis through the steroid hormone aldosterone. Finally, the adrenal gland secretes large amounts of the androgen precursor dehydroepiandrosterone sulfate (DHEAS), which plays a major role in fetoplacental estrogen synthesis and as a substrate for peripheral androgen synthesis in women.
The adrenal glands are bilateral structures located immediately above the kidneys (ad, near; renal, kidney) (
Fig.
43.1 ). In humans they are also referred to as the suprarenal glands because they sit on the superior pole of each kidney. The adrenal glands are similar to the pituitary in that they are derived from both neuronal tissue and epithelial (or epithelial-like) tissue. The outer portion of the adrenal gland, called the adrenal cortex (
Fig. 43.2 ), develops from mesodermal cells in the vicinity of the superior pole of the developing kidney. These cells form cords of epithelial endocrine cells. The cells of the cortex develop into steroidogenic cells (see ). In adults the adrenal cortex is composed of three zones—the zona glomerulosa, the zona fasciculata, and the zona reticularis—that produce mineralocorticoids, glucocorticoids, and adrenal androgens, respectively (see
Fig. 43.2B
Soon after the cortex forms, neural crest–derived cells associated with the sympathetic ganglia, called chromaffin cells, migrate into the cortex and become encapsulated by cortical cells. Thus the chromaffin cells establish the inner portion of the adrenal gland, which is called the adrenal medulla (see
Fig. 43.2). The chromaffin cells of the adrenal medulla have the potential to develop into postganglionic sympathetic neurons. They are innervated by cholinergic preganglionic sympathetic neurons and can synthesize the catecholamine neurotransmitter norepinephrine from tyrosine. However, high levels of cortisol that drain into the medulla from the adrenal cortex induce expression of the enzyme phenylethanolamine N-methyl transferase (PNMT), which transfers a methyl group onto norepinephrine to produce the catecholamine hormone epinephrine, the primary hormonal product of the adrenal medulla (see
Fig. 43.2B).
CHAPTER 43 The Adrenal Gland 767 • Fig. 43.1 The adrenal glands sit on the superior poles of the kidneys and receive a rich arterial supply from the inferior, middle, and superior suprarenal arteries. The adrenals are drained by a single suprarenal vein. (Modified from Drake RL et al. Gray’s Anatomy for Students. Philadelphia: Churchill Livingstone; 2005.)